The invention relates generally to the disposal of solid wastes, and in particular to the disposal of wood wastes by gasification.
The disposal of solid waste materials has become a major problem in this country. In the Southern California area, for example, over 60,000 tons of solid waste materials must be disposed in landfills every day. As the amount of solid waste increases and the number of usable landfill sites decreases, the problem of solid waste disposal rapidly approaches crisis proportions.
A major component of solid waste materials is wood waste. For example, of the 60,000 tons per day of solid waste generated in Southern California, over 5,000 tons per day are wood wastes. Wood wastes, being organic and being generally free of metal and other contaminants, offer the possibility of alternative disposal and energy generation through some form of incineration. Incineration devices for wood waste are known in the art. However, such devices have been considered impractical for large-scale, continuous disposal of large varieties of wood wastes because of the considerable expense, operating difficulties and/or air pollution normally associated with the use of such devices.
Instead of incinerating wood wastes by direct, open-air burning, it has been proposed to incinerate wood wastes by "gasifying" them to produce a low BTU synthesis gas. Gasification, in theory, would produce little air pollution. Also, the production of usable synthesis gas in the gasification process would theoretically reduce the overall costs. However, the considerable operating problems inherent in building and operating a practical large scale device capable of such gasification have, until now, gone unresolved.
Batch-wise gasification operations have been found to be slow, inefficient and incapable of generating a constant flow of consistent quality synthesis gas.
Continuous gasification operations are generally more efficient and consistent, but they entail complex operating difficulties. Fluidized bed reactors, for example, have been tried, but have generally been found to be unsatisfactory. It is very difficult to monitor and maintain feed input within the narrow control ranges required for fluidized bed operation, and it is very difficult to rigorously separate the synthesis gas from the fluidized solid material.
Downwardly gravitating fixed bed reactors have also been tried. They have been found to be easier to operate than fluidized bed reactors, but unsolved operating difficulties remain. Most prominent of these operating difficulties is the necessity in fixed bed operations to somehow maintain continuous control of the gravitating solid bed of non-homogeneous wood waste particles in varying stages of combustion.
For example, non-homogeneous wood waste particles in a moving solid bed exhibit a tendency of the particles to interlace and bridge, thereby inhibiting downward flow. The breakup of such conglomerated material often requires considerable force. Due to the void area formed below such a bridge, the sudden break-up or collapse of the bridge often results in the rapid formation of uncontrolled channeling, fluidizing and "rat-holing" which result in unstable operation. Bridging may occur on a small scale (a matter of inches) or on a large scale (traversing the entire bed).
A secondary and compounding effect of bridging is the resultant localized increased exposure of carbonaceous tar to localized high temperatures which can result in the rapid fusing of the particles to form large balls of slag ("clinkers"). Such slag promotes additional bridging.
Some localized bridging is unavoidable. The problems of operating a continuous gasification operation reduces to formulating a system which can successfully maintain solid bed reaction control despite such bridging.
No known prior art device has been able to satisfactorily deal with the problem of bridging and "clinkers" in a fixed bed continuous wood gasification operation. Continuous mixing or agitation of the bed has been recommended. However, it has been found that continuous mixing or agitation often promotes packing below the point of agitation due to the downward forces which increase vessel differential pressures (similar to the way that differential pressures develop across a clogged filter). The resulting high pressure continues to increase until a localized "blowout" occurs, whereupon temporary fluidizing and loss of particle bed control result.
Another problem inherent in the use of continuous gasification operations is the handling and disposal of tars and other high molecular weight materials which form in the hot, reducing atmosphere within the gasification reactor. Tar and other high molecular weight materials tend to condense downstream of the gasification reactor and plug up downstream equipment and piping. This is especially true in updraft (counter-current) continuous gasification devices.
There is therefore a need for a large scale continuous wood particle gasification system which can be continuously controlled to efficiently reduce non-homogenous particles of wood waste to a relatively tar-free fuel gas (without resulting in the release of large quantities of air pollutants).